1. Field of the Invention
The present invention generally relates to a wafer bonding method and, more particularly, to a method using thermal ultra-sonic energy to activate a wafer surface and achieve direct bonding between wafers at low temperatures so as to save time and achieve environmental protection using such a lead-free process. The present invention is suitable for use in the GaN-based white light-emitting diode (LED) industry.
2. Description of the Prior Art
The purpose of the LED was mainly for an alert signal or advertisement. However, the LED has attracted more attention due to its possible use as a light source for the displays since the realization of the high power white LEDs.
The conventional white LED is realized using the blue GaN LED to emit blue light, and the blue light is then converted into white light by fluorescent powders. GaN is usually grown by hetero-epitaxy on, for example, a sapphire substrate because a large-area GaN substrate is still unavailable. However, the sapphire substrate has poor electrical conductivity and poor thermal conductivity. When the temperature of the LED is high enough to cause reduction in light-emitting efficiency, the poor thermal conductivity of the sapphire substrate limits the driving current for the LED. On the other hand, because of the poor electrical conductivity of the sapphire substrate, the electrode for the p-type and the electrode for the n-type are required to be formed on the same side of the substrate, which requires higher packaging cost and results in smaller transparent area. Also, the hard but fragile sapphire substrate is incompatible with conventional packaging processes. Therefore, the LED structure has to be mounted onto a substrate with excellent electrical conductivity and thermal conductivity using wafer bonding, and then the sapphire substrate is removed using laser left-off so to increase the light-emitting efficiency of the white GaN LED by up to 95%.
There have been reported several wafer bonding methods corresponding to different specifications, and the methods can be categorized into three divisions according to process temperatures and whether a medium layer is needed or not.
1. Anodic Bonding:                Please refer to FIG. 1, wherein the anode and cathode are coupled to a Si wafer 11 and a glass substrate 12 comprising ions, respectively. The anode is coupled to a high voltage supply 14 (up to 1,000V) through a wire 13 so as to generate a static electric field induced by the mobile ions in the glass substrate 12. A heater 15 is disposed under the substrate 12 to heat up to 400° C.˜450° C. with an applied pressure 16 so as to achieve wafer bonding. However, this method is problematic that the process temperature is so high that different thermal expansion coefficients of the Si wafer 11 and the glass substrate 12 result in a great thermal stress during temperature lowering to cause undesirable bending and crack of the Si wafer 11. On the other hand, the glass substrate 12 has to be electrically conductive.        
2. Glass Frit Bonding:                Please refer to FIG. 2A and FIG. 2B, wherein screen-printing 23 is used to provide a colloidal glass frit 22 with low melting point on a substrate 21. Pre-cure is used to remove the solvent in the colloidal glass frit 22, a pressure 26 (about 1 bar) is applied, and then the substrate 21 is baked at 400° C. to 450° C. so as to achieve wafer bonding. However, this method is problematic that process temperature is high, and the bonding quality is unsatisfactory.        
3. Direct Bonding:                Direct bonding is the most adopted approach in the industry. It employs oxygen atoms to react with the Si atoms on the surface of a Si wafer to form Si—O—Si bonds that provide the required bonding. Requirements in surface roughness and cleanness for this method are very strict and therefore a specially designed clean room is necessary for the pressurizing and heating process. Thus, the method is problematic because of:        (1) high equipment cost;        (2) long time for bonding—the whole process including heating, pressurizing, and annealing takes as long as two hours;        (3) high temperature for bonding—the process takes place at 450° C.; and        (4) high temperature for annealing—the process takes place at 1000° C.;        
Therefore, there exists a need in providing a wafer bonding method using thermal ultra-sonic energy to activate a wafer surface and achieve direct bonding between wafers at low temperatures (100° C. to 200° C.) so as to save time (within 20 seconds) and achieve environmental protection using such a lead-free process.